Handsets or cell phones are commonplace in society. The handsets are equipped with an antenna that sends outgoing signals and receives incoming signals. One important element in the handset is a power amplifier that is typically connected to the antenna. The power amplifier is utilized to receive signals (e.g., signals representing voice or sound) and to amplify these signals to a level suitable for transmission through the antenna. Cell phones typically operate at frequencies in the range between 1 GHz and 2 GHz.
Each country regulates the operating characteristics of handsets that are allowed to operate in that country. For example, many countries regulate the maximum amount of power that devices (e.g., handsets) are allowed to emit in adjacent channels (i.e., channels that are not the intended operating channel of the device). The amount of undesired or unwanted power emitted in adjacent bands is measured by using a ratio of the amount of power in the intended or specified channel divided by the amount of power emitted in channels adjacent to the intended channel, which is referred to as “adjacent channel power ratio” (ACPR). Each country can have an appropriate regulatory body specify the minimum acceptable ACPR for a device before approval of that device is granted.
The performance of the power amplifiers at the highest power output is dependent on the load impedance of an output transistor. Generally, it is desired that the amplifier operate in the linear region of operation, where the gain of the amplifier varies in a generally linear fashion. However, an unfavorable load impedance can cause the amplifier to operate in the saturation region that adversely affects or degrades the amplifier's performance. Unfortunately, this load condition is difficult to control in a wireless environment especially in the case of mobile communication because power reflected back to the antenna and the power amplifier can change the load impedance seen by amplifier, which in turn can adversely affect the performance of the amplifier. Specifically, power transmitted by the handset can be reflected back to the handset by the presence of metal objects in the environment (e.g., bars in a concrete wall beside a person placing the call) and eventually back to the amplifier, thereby affecting the load impedance.
One approach is to employ an isolator between the power amplifier module and the antenna to ensure a well-controlled impedance for the power amplifier. Unfortunately, this added component incurs both cost and space for mobile phone handsets. Furthermore, current isolator technology is incompatible with integrated circuit technology and packaging technology, thereby excluding the possibility of integrating the isolator with other components in the power amplifier. Consequently, the isolator is contrary to and militates against the trend to increase the functionality of electronic devices and components.
One approach to enable a power amplifier to accommodate a wide range of loads is described in “A Low Distortion and High Efficiency Parallel-Operation Power Amplifier Combined in Different Phases in Wide Range of Load Impedances,” by H. Ikeda, H. Kosugi and T. Uwano, IEEE Microwave Technologies and Techniques Symposium (MTT-S), pages 535 to 538, 1996. Ikeda et al. propose the use of two or more power amplifiers with equal but phase-shifted signal paths. By diverting the reflected power into to different paths, where one of the paths includes a phase delay, the performance of the power amplifier may be improved. This approach is known as “balanced power amplifiers.”
Unfortunately, the implementation proposed by Ikeda et al. uses 3 dB hybrids, Wilkinson combiners, and delay lines, that result in large physical sizes. Consequently, the large area required by the Ikeda implementation makes such an implementation unsuitable for miniature power amplifier modules that are utilized in may mobile telephone handset applications.
Stated differently, a primary disadvantage of this approach is that the approach requires components that are relatively large. Consequently, this approach is not conducive for space-efficient designs, which is the current trend. For example, as the handsets decrease in size or as more functions are integrated into the handsets, the space that is allocated to power management and control is rapidly decreasing.
Based on the foregoing, there remains a need for a mechanism to reduce the size or area occupied by the balanced power amplifier so that the balanced power amplifier can be utilized in miniature, space conserving applications and to further overcome the disadvantages set forth previously.